Grant the new user Full access permission to the old mailbox in exchange, then to add the mailbox to the new users Outlook right click where it says 'Mailbox - New persons name' in the left hand pane > then choose properties > advanced > advanced > then click add and type the name of the old persons mailbox. Keep clicking ok until you are back to the inbox and the mailbox will be added.

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The MCTS: Microsoft Exchange Server 2010 certification validates your skills in supporting the maintenance and administration of the Exchange servers in an enterprise environment. Learn everything you need to know with this course.